A meta-analysis was carried out on the basis of 155 research studies on school effectiveness, comprising a total of 1.211 associations between school effectiveness enhancing factors and student outcome variables. The original studies were carried out between 1984 and 2005. The school effectiveness enhancing conditions that were included were: cooperation between staff, an orderly school climate, monitoring, curriculum quality, homework, learning time, parental involvement, achievement orientation, educational leadership and differentiation. The outcome variables were student achievement results in Mathematics, mother tongue language and other subject matter domains, including Science. A multi level approach to meta-analysis was used, on the basis of which numerical effect sizes (Fischer’s Z coefficients) were calculated. Effect sizes for the curriculum related factors, curriculum quality and teaching time were relatively the highest (,15), closely followed by the school climate factors (orderly climate and achievement orientation (,14). According to widely accepted standards (Cohen, 1969), these effects are to be considered as small effects; although in the discussion some arguments are provided that might lead to an upgrading of what one could call the “practical significance” of these effect sizes. The analyses included moderator variables representing study characteristics that were analyzed for their influence on the effect sizes of the various school effectiveness enhancing factors. For most factors, effect sizes were slightly higher for studies carried out in primary schools as compared to secondary schools. For other moderator variables, such as subject matter area, the results were less straightforward. Results are discussed for their substantive educational implications, and their meaning for the field of empirical school effectiveness research.